This invention is a continuation-in-part of an improved process for thermally converting methane into hydrocarbons with higher molecular weights.
Among the methane sources are natural gases and refinery gases. The natural gases can either be gases associated with crude oil or not; their composition varies rather noticeably according to their origin, but they generally contain a volume percentage of methane from 60 to 95%. This methane is always associated with other higher alkanes up to and even exceeding the C.sub.6. Various cryogenic processes serve to separate the gases into several fractions once the water and acid components are removed. Such fractions are: nitrogen, liquefied natural gases the propane and butane fraction of which is separated, and a fraction essentially composed of methane associated with a low amount of ethane. The latter fraction is either re-injected into the well in order to maintain the pressure which makes the crude oil rise or fed through a gas pipe line as a combustible gas, or else subjected to flaring.
Other sources of methane are refinery gases which have various origins: crude oil first still gases, hydroreforming gases, different hydrotreatment gases, thermal cracking gases, catalytic cracking gases; all these gases contain, in various proportions, methane associated with numerous other gaseous constituents, such as light hydrocarbons, nitrogen, CO.sub.2, hydrogen, etc.
For example an effluent gas from a fluidized bed catalytic cracking unit comprises, after washing, about 30% by volume of methane. This gas is often fractionated by cooling under pressure, resulting in two fractions, one containing hydrogen, nitrogen, methane and a low amount of ethylene, the other fraction being composed of the main part of the initial ethylene, ethane, propane and propylene. The latter fraction can be advantageously fed into a DIMERSOL type unit, whereas the first one is fed back into the fuel-gas system of the refinery, where it is used as a fuel.
The conversion of methane into hydrocarbons with higher molecular weights is doubtlessly interesting; in far away sources of natural or associated gases, the conversion of methane into acetylene, ethylene and aromatic compounds can, using sequences of well-known processes, result in liquid fractions that are easier to transport and/or to valorize.
As an example, after separating the possibly formed solids, the fraction of aromatic compounds can be separated, the gaseous fraction can first be treated in units allowing the oligomerization and/or the cyclization of acetylene, and then, after another gases/liquids separation, the residual gas fraction with a high ethylene content can be treated in Dimersol type units which produce ethylene oligomers.
The conversion of methane, even partially achieved, on the refining scene proper, into products which are easier to valorize, also shows considerable economical advantages.
Different methods for converting methane have already been suggested; for example, U.S. Pat. No. 4,199,533 describes a method allowing the production ethylene and/or ethane from methane, which consists of reacting chlorine with methane at a temperature higher than 700.degree. C. This process has an important drawback since it involves the use of very corrosive gases such as chlorine and hydrochloric acid at high temperatures.
In addition, Patent FR-A-711,394 describes a process for transforming methane in which the heat necessary for the heating is obtained from starting points of a gas production process.
Patent FR-A-1,364,835 describes a process preventing side reactions in the hydrocarbon oxidation.
Note that the previous techniques are also illustrated by Patents WO-A-8,700,546 and DE-A-1,542,406.
Many processes for the catalytic cracking of methane have been described in the prior art, using for example zeolitic catalysts, as in Patent EP No. 93543, but all the catalysts used show a very short life, which is due to coke layers formed in the reaction.
The oxidizing coupling of methane is a well-known process which can be achieved either in the presence of oxygen or even in the absence of oxygen; in that case, metallic oxides intervene in the reaction by being reduced; U.S. Pat. Nos. 4,172,810, 4,239,658, 4,443,644 and 4,444,984 are examples for this type of processes. They are discontinuous since the metallic oxide must be regenerated.
Among the thermal cracking processes which can transform methane is the "Wulff process", that consists in using refractory contact masses; at first, the refractory mass is heated by air combustion of a fuel which can be the feedstock itself; then, the hydrocarbon to be cracked is decomposed by absorbing the heat accumulated by the refractory material during the previous period; it is thus a discontinuous process.
The electric arc and plasma processes are essentially directed to the preparation of acetylene; their high electric power consumption makes them difficult to exploit.
Another type of process, which is sometimes called autothermal, consists in burning a part of the feedstock in order to supply the cracking reaction with the necessary calories; this type of process uses a burner in which about 1/3 of the hydrocarbon is burnt, the rest being cracked. Considering the high temperatures that are employed, this type of process essentially produces acetylene and coke.
Patent FR No. 1,211,695 describes a combined process of hydrocarbon pyrolysis that consists of mixing methane with warm combustion gases which do not contain oxygen in excess, then in injecting into the obtained mixture paraffinic hydrocarbons with more than one atom of carbon; a very low amount of the methane can be transformed into acetylene with this process.
The dehydrogenating thermal coupling of methane is highly endothermic and requires the obtaining of a very high thermal flow density at high temperatures, from 1,100.degree. to 1,500.degree. C. It is necessary that the maximum heat supply is performed in the zone where the endothermic cracking and dehydrogenation reactions take place; besides, the obtaining of valorizable products such as acetylene, ethylene and/or aromatic compounds requires a very short contact time followed by a rapid quenching so that a "square" temperature profile can be obtained.
There is presently no industrial process available using a controlled heat transfer through a wall, so as to transform methane into easily valorizable hydrocarbons.